1. Field of the Invention
The present invention relates to an air-fuel ratio control system for an internal combustion engine.
2. Description of Background Information
In internal combustion engines provided with a three-way catalytic converter in the exhaust system, it is customary to use an air-fuel ratio control system in which the air-fuel ratio of the mixture supplied to the engine is controlled around a stoichometric value (14.7 : 1 for example) according to the composition of the exhaust gas and operating conditions of the engine. This is because optimum operation of the three-way catalytic converter is enabled at the stoichometric air-fuel ratio. As an example of this type of air-fuel ratio control system, Japanese Patent application No. 57-217548 discloses an air-fuel ratio control system of the air intake side secondary air supply system type which is constructed such that an oxygen sensor is provided in an exhaust system of the engine, and the feedback control of the air-fuel ratio is performed such that an amount of air intake side secondary air flowing through an air intake side secondary air supply passage leading to the downstream side of the throttle valve is varied in response to the output signal of the oxygen sensor.
However, in this conventional air-fuel ratio control system having the feature of the supply of the air intake side secondary air, there was a tendency for the operation of the system during the feedback control under a high load condition to become inaccurate. Under such a condition, the magnitude of the vacuum downstream of the throttle valve, i.e., the intake manifold, becomes small. Therefore, the amount of the secondary air introduced into the intake manifold per unit time becomes smaller than the corresponding amount under the light or medium load condition even though the sectional area of the air intake side secondary air supply passage is constant. As a result, the air-fuel ratio of the mixture becomes over-rich and the amount of the noxious component, especially CO (carbon monoxide) and HC (hydrocarbon), emitted through the exhaust gas is increased.